1. Field of the Invention
The present invention relates to a position detection apparatus for detecting a position of movement of two members which move relative to each other, and an arithmetic processing unit used for position detection.
2. Description of the Related Art
Heretofore, a position detection apparatus for detecting a position of movement of two members which move relative to each other is well known. This position detection apparatus comprises a scale on which a periodic signal whose signal level varies in a certain wavelength is recorded along a certain direction, a head section for detecting the periodic signal recorded on the scale, and an arithmetic processing section for performing signal processing of the periodic signal detected by the head section to output position information. The scale and the head section are fitted to a moving element and a base element of two members moving relative to each other. The position detection apparatus detects with the scale the periodic signal whose signal level changes, and supplies the detected periodic signal to the arithmetic processing section. The arithmetic processing section outputs position information showing a position of relative movement of the two members, based on the periodic signal detected by the head section.
With such a position detection apparatus, in order to further interpolate a recorded waveform of the periodic signal recorded on the scale to perform position detection with high resolution, the detected periodic signal is subjected to polar conversion to form an angle signal, and position information is formed using the angle signal.
The angle signal obtained by polar conversion is, as shown in FIG. 1, a signal having a modulo phase in which signals in the angle range of from 0xc2x0 to 360xc2x0 are repeated over many rounds. With a conventional position detection apparatus, arithmetic processing and interpolation processing can be easily performed using this angle signal, enabling efficient processing.
With a conventional position detection apparatus, it is necessary to provide a low pass filter in order to remove internal noise such as detection noise by means of the head section, quantization noise at the time of A/D conversion, quantization noise at the time of polar conversion or the like. However, when the conventional low pass filter is applied to the angle signal, filtering cannot be performed precisely. For example, if a portion jumping from 360xc2x0 up to 0xc2x0 is smoothed, the conventional low pass filter regards the smoothed portion as an angle change from 360xc2x0 up to 0xc2x0, hence, the output thereof is a value in the vicinity of 180xc2x0 on the contrary, as shown in FIG. 2.
Therefore, with a conventional position detection apparatus, the angle signal is converted to a signal which can output an angle larger than 360xc2x0, instead of a signal which repeats from 0xc2x0 to 360xc2x0, to thereby perform filtering, or filtering is performed with respect to a periodic signal before the polar conversion. However, when the angle signal is converted to a signal which can represent an angle larger than 360xc2x0, a considerably large operation table is required, resulting in deterioration in the arithmetic operation and cost increase. Moreover, when filtering is performed with respect to a periodic signal before the polar conversion, noise occurring after the polar conversion cannot be removed. As a result, position information with high precision cannot be obtained.
It is an object of the present invention to provide a position detection apparatus that can remove a noise component from an angle signal obtained by performing polar conversion with respect to a position signal, and an arithmetic processing unit that can remove a noise component from the angle signal.
The position detection apparatus according to the present invention comprises: a recording medium on which a position signal comprising a periodic signal is recorded; a detection section comprising a first detection head which moves relative to the recording medium along the recording direction of the position signal for detecting the position signal, and a second detection head which is disposed apart from the first detection head by a predetermined distance in the recording direction of the position signal, and moves relative to the recording medium, operating together with the first detection head for detecting the position signal; a polar conversion section for converting the position signal detected by the first detection head and the second detection head into an angle signal showing a relative position of the recording medium and the detection section in one period as an angle; a low pass filter for removing high pass component in the angle signal output from the polar conversion section; and an output section for outputting relative position information of the recording medium and the detection section, based on the angle signal in which the high pass component has been removed by the low pass filter.
With the position detection apparatus, low pass filtering is performed with respect to the angle signal obtained by performing polar conversion.
Moreover, with the position detection apparatus according to the present invention, the low pass filter has a frequency control oscillator for outputting a periodic signal in which the frequency is controlled based on a frequency control signal; a phase comparator for comparing the phase of the angle signal output from the polar conversion section and the periodic signal output from the frequency control oscillator to thereby output a phase error; an integrator for integrating the phase error output from the phase comparator to thereby output a velocity error; and an adder for adding the velocity error output from the integrator and the phase error output from the phase comparator to thereby generate the frequency control signal, characterized in that the frequency control oscillator controls the frequency of the periodic signal so that the phase error is zero based on the frequency control signal, and outputs the periodic signal as the angle signal in which the high pass component has been removed.
According to the position detection apparatus, the low pass filter determines a phase error between the input angle signal and the angle signal to be output, and outputs an angle signal having a frequency such that the phase error is zero.
An arithmetic processing unit according to the present invention comprises: a polar conversion section for respectively converting a first periodic signal and a second periodic signal whose phase is different from that of the first periodic signal into an angle signal showing an angle in one period of the first periodic signal and the second periodic signal; a low pass filter for removing a high pass component in the angle signal output from the polar conversion section; and an output section for outputting position information shown by the first periodic signal and the second periodic signal, based on the angle signal wherein the high pass component has been removed by the low pass filter.
According to this arithmetic processing unit, low pass filtering is performed with respect to the angle signal obtained by performing polar conversion.
Moreover, with the arithmetic processing unit according to the present invention, the low pass filter has: a frequency control oscillator for outputting a periodic signal in which the frequency is controlled based on a frequency control signal; a phase comparator for comparing the phase of the angle signal output from the polar conversion section and the periodic signal output from the frequency control oscillator to thereby output a phase error; an integrator for integrating the phase error output from the phase comparator to thereby output a velocity error; and an adder for adding the velocity error output from the integrator and the phase error output from the phase comparator to thereby generate the frequency control signal, characterized in that the frequency control oscillator controls the frequency of the periodic signal so that the phase error is zero based on the frequency control signal, and outputs the periodic signal as the angle signal in which the high pass component has been removed.
With the arithmetic processing unit, the low pass filter determines a phase error between the input angle signal and the angle signal to be output, and outputs an angle signal having a frequency such that the phase error is zero.
With the position detection apparatus and the arithmetic processing unit according to the present invention, low pass filtering is performed with respect to the angle signal obtained by performing the polar conversion. Thereby, with this position detection apparatus, a noise component included in the angle signal can be removed. As a result, not only the noise occurring in the detection section but also quantization noise or the like at the time of polar conversion can be removed.
Moreover, with the position detection apparatus and the arithmetic processing unit according to the present invention, the low pass filter determines a phase error between the input angle signal and the angle signal to be output, and outputs an angle signal having a frequency such that the phase error is zero. Thereby, with this position detection apparatus, low pass filtering that can follow the change from 360xc2x0 to 0xc2x0 is performed with respect to the angle signal changing from 360xc2x0 to 0xc2x0, enabling removal of noise.